In pulverized-fuel fired power plants, the wet fuel is generally dried with the help of stack gases prior to being fed into the furnace of the boiler. The drying of the fuel is necessary, because burners of pulverized fuel require dry fuel for stable combustion. In fluidized-bed boilers, the fuel need not be dry for efficient combustion, but rather, drying and combustion can take place in the fluidized bed, whose heat content is relatively high.
In drying with stack gases, the stack gases and the steam generated in the drying process will become mixed with each other. Heat recovery from the gaseous mix of the stack gases and the steam emerging from the drying process in not generally economically feasible, because the heat of condensation cannot be recovered at a sufficiently high temperature, and moreover, the acidic components (NO.sub.x and SO.sub.x) of the stack gases cause strong corrosion of the heat exchanger surfaces at temperatures below the dew point of water.
The fuel can be dried for pulverized fuel firing also with the help of different steam-heated dryers in which the heat for drying is obtained from steam fed to the dryer. The steam is condensed on heat transfer surfaces designed into the dryer. Typically, low-pressure steam is used at a temperature as low as possible, and the steam released from the fuel is not necessarily recovered.
An advantageous steam-heated dryer construction known in the art is a fluidized-bed steam-heated dryer in which the pressure level of dryer exhaust steam is first elevated by means of a compressor and the compressed steam is then introduced to the steam-condensing surfaces of the dryer, whereby the heat of condensation can be recovered from the exhaust steam. The drawbacks of such a dryer are its high investment costs and relatively high internal power consumption of the compressor.
The German application publication DE 3,726,643 discloses a construction whose use is limited to circulating fluidized-bed boilers, in which the entire flow of the circulating bed solids is routed to the mixer-type dryer. Typical to commercial circulating fluidized-bed boilers, also this system uses a heat exchanger construction with cooling surfaces as the dryer. In the embodiment described in the DE application publication, recirculated steam acts as the fluidizing gas. The dryer is provided with heat transfer surfaces, because the amount of recirculated bed solids cannot be controlled to match the required drying effect. Cooling of the recirculated bed solids thus takes places in three different stages: drying of fuel, superheating of recirculating steam and heat transfer to cooling tubes placed in the dryer bed.
The abovedescribed system still has the drawback of complicated dryer construction and process arrangement, which cause high investment costs. Furthermore, for efficient heat transfer, the bed temperature of the dryer in such an embodiment must be clearly higher (by 100.degree.. . . 300.degree. C.) than the phase transition temperature necessary for evaporation of water into steam, whereby fuel gasification and tar formation may hamper the technical feasibility of the apparatus.